The present invention relates to a process for producing a poly-silicon (hereinafter abbreviated as poly-Si) film for liquid crystals and semiconductor devices, and a method for inspecting the poly-silicon film.
The reason why a poly-silicon film is superior to an amorphous silicon (a-Si) film as the active layer of a thin film transistor (TFT) used as a driver element in a liquid crystal display is as follows: in the case of the poly-silicon film, since the mobility of a carrier (electrons in n channel or holes in p channel) is high, the cell size can be reduced, so that the precision and minuteness of the liquid crystal display can be enhanced. In addition, the formation of a conventional poly-Si TFT requires a high-temperature process at 1,000° C. or higher. On the other hand, a TFT having a high carrier mobility can be formed in a low-temperature process permitting employment of an inexpensive glass substrate, when there is adopted a low-temperature poly-silicon formation technique in which annealing of only a silicon layer with a laser does not make the temperature of the substrate high.
In this laser annealing, as shown in FIG. 13, an a-Si film formed on a glass substrate is scanned while being irradiated with light absorbable thereby, to make the whole a-Si film into a polycrystal, whereby a poly-Si film is obtained. As shown in FIG. 14, the poly-Si grain size varies with the surface density of irradiation energy (fluence) of a laser, so that the stability of the laser reflects on the grain size distribution of the poly-Si. The carrier mobility of the poly-Si film increases with an increase of the grain size. In order to attain high TFT characteristics with in-plane uniformity, it is necessary to make the grain size distribution uniform and maintain a large grain size. To attain the large grain size, employment of a fluence in the D region shown in FIG. 14 is sufficient. However, if the fluence shifts upward owing to the instability of the laser, or the like, the fluence enters a region shown as the E region in FIG. 14, i.e., a region where the poly-Si film contains micro crystals with a grain size of 200 nm or less. In this case, the carrier mobility is decreased, resulting in a faulty device. The grain size varies not only with the laser fluence but also with the nonuniformity of thickness of the a-Si film before the laser annealing. Therefore, in order to form the poly-Si film so that its grain size may always be in a definite range, the laser instability and the thickness change of the substrate have to be kept slight. For this purpose, control of the grain size is necessary. Accordingly, it becomes important to control the poly-Si grain size to keep it constant, by checking the poly-Si grain size and feeding back the check result to the laser annealing conditions.
As a method for the control, measuring the grain size itself of the poly-Si is the most reliable. The grain size has been measured by incorporating a sample for the check into an initial or intermediate production lot, or by randomly sampling a product and directly observing the grain size of a poly-Si film formed in a production process, by an electron microscope or a scanning tunnel microscope. As other prior arts, there are the following methods. Japanese Patent. Kokai No. 10-214869 discloses a method in which a poly-silicon film is evaluated on the basis of its transmittance. According to this method, the grain size cannot be estimated, though insufficient crystallization due to the insufficient fluence of laser beams can be monitored on the basis of the ratio between a-Si and a poly-Si by utilizing the difference in absorption coefficient between a-Si and the poly-Si. Japanese Patent Kokai No. 11-274078 discloses a method in which a poly-silicon film is evaluated on the basis of its surface gloss (reflectance). In this method, the change of the gloss with the poly-Si grain size is utilized and the gloss is considered to be minimal at an optimum poly-Si grain size. This optimum poly-Si grain size corresponds to a grain size at which the reflectance becomes minimal, namely, the surface roughness becomes maximal.